Others will tell you to get the proper Tandem Forks but they can be expensive but do work. There are alternatives within the Mountain bike world. Or there would be- providing you can get hard enough springs for them. Unless you can get a hard enough spring- then they will be bouncy.

I do ride my Tandem aggressively offroad and Initially I used Marzochhi Freeride forks. Hardest springs available and they still bounced a lot. Quick cure though was to use the heavist weight Oil I could get (20W) and to overfill the forks to slow down the action. Still bounced though. Then I went Rockshox Boxers and used the heaviest springs and oil again. These were better but had to slow the rebound and put in the maximum amount of Sag adjusters in-15 in each leg- before they stop being a Pogo stick. They do work well and work well for our 400lbs team and the lumpy terrain that we ride on.